The installation of an oxidation catalyst in an exhaust passage to purify the soluble organic fraction (SOF), hydrocarbons (HC) and carbon monoxide (CO) from particulate matter (PM) in the exhaust of a diesel engine, is disclosed in for example the Toyota Technical Review Vol. 43, No. 1 published in May, 1993.
FIGS. 9A-9C show one example of such a diesel engine oxidation catalyst 13. The catalyst 13 comprises a honeycomb type substrate 14 of cordylite, and a wash coat 15 supported on the substrate 14, as shown in FIGS. 9A and 9B. The wash coat 15 comprises an alumina (Al.sub.2 O.sub.3) base material, and platinum (Pt) or palladium (Pd) adsorbed on the base material. The substrate 14 and wash coat 15 comprise a plurality of longitudinal partitions 16 as shown in FIG. 9C, and exhaust gas passes through spaces enclosed by these partitions 16.
The main components of PM are oil in fuel, SOF due to unburnt fuel, dry soot produced by dehydration of fuel, and sulfate (SO.sub.4) due to sulfur in fuel. The dry soot and sulfates comprise an insoluble organic fraction (ISF).
The proportions of these components vary according to the engine combustion system. In the case of a precombustion chamber type diesel engine:
SOF=10-40%, dry soot=60-90%, sulfates=1-3%.
In the case of a direct injection type diesel engine:
SOF=30-60%, dry soot=40-70%, sulfates=1-3%.
The performance of an oxidation catalyst varies according to the catalyst inlet temperature, as shown in FIGS. 11A-11C. A Pt type oxidation catalyst has a high oxidizing activity for HC as shown in FIG. 11A, but the same is true for SO2. Therefore, when the exhaust temperature is high, sulfates which are oxidation products of SO.sub.2 are produced in large quantity as shown in FIG. 11C, and the decrease of PM at high temperature is consequently lessened as shown in FIG. 11B.
A Pd type catalyst, on the other hand, has a low oxidizing activity for HC and SO.sub.2. Hence, even when the temperature increases, not much sulfate is produced as shown in FIG. 11C, and the reduction of PM due to decrease of SOF is therefore maintained even at high temperature. These catalysts reduce PM and SOF even at exhaust temperatures where oxidizing activity is low, as shown in FIG. 11B, but this is due to the fact that PM and SOF are adsorbed on the catalyst and build up on it.
FIGS. 12A and 12B compare a Pt type oxidation catalyst with a Pd type oxidation catalyst regarding the amount of dry soot and SOF which are adsorbed or built up on the catalyst when the exhaust temperature conditions are varied.
When using a Pt type oxidation catalyst, at the exhaust temperature of 200.degree. C. or higher, the amount of dry soot built up decreases, and at 300.degree. C. or higher, there is almost no build-up. Using a Pd type oxidation catalyst on the other hand, since its oxidizing activity is low, dry soot build-up begins to decrease only at 300.degree. C. or higher. SOF adsorption also depends on the exhaust temperature, but not on whether the catalyst has high or low activity, and at 200.degree. C. or over, there is practically no SOF adsorption on the catalyst. This is due to the fact that SOF vaporize at 200.degree. C. or higher temperatures.
FIG. 13 shows a conversion rate with respect to temperature of a Pt type oxidation catalyst on which PM (SOF) has been adsorbed or built up at a low exhaust temperature at which the catalyst has a low oxidizing activity. It is seen from this figure that adsorption or build-up of PM (SOF) largely decreases oxidizing activity relative to HC and CO. When the temperature reaches 200.degree. C. and over, CO is converted since SOF which had been adsorbed or built up is vaporized and released from the catalyst. When the temperature reaches approximately 280.degree. C., SOF which had been adsorbed or built up suddenly burns, restoring the oxidizing activity of the catalyst with respect to HC and CO. Hence, catalyst performance relative to each component is largely affected by the exhaust temperature.
FIG. 14 shows an exhaust temperature frequency in the 10/15 running mode of a vehicle carrying a direct injection type diesel engine. It is seen that the time required to reach the oxidizing activity temperature is less at the downstream under-floor position than in the upstream exhaust manifold position.
FIG. 15A shows the relation between the capacity of a Pt type oxidation catalyst and PM adsorption/build-up performance at an exhaust temperature of 150.degree. C. where the oxidizing activity is low. FIG. 15B shows this relation in more detail. FIGS. 16A and 16B are similar graphs but showing a PM oxidation performance at an exhaust temperature of 250.degree. C.
From these graphs, it is seen that PM adsorption/build-up performance increases the higher the catalyst capacity. It is also seen that although oxidizing activity increases to some extent with increase of catalyst capacity, it is steady above a certain capacity.
FIG. 17A is a graph showing the relation between catalyst capacity and rate of sulfate increase for a conventional Pt type oxidation catalyst. FIG. 17B shows sulfate increase rate characteristics when space velocity (SV) is varied for a Pt type oxidation catalyst. As the space velocity increases for the same exhaust flowrate if the catalyst capacity is kept low, increase of sulfate can be suppressed. In practice however, a certain catalyst capacity is required to enhance HC and CO oxidizing activity, hence the oxidizing activity and sulfate suppression performance are in a mutually conflicting relationship with one another.
Oxidation catalysts for diesel engines therefore have the above characteristics. In view of the need to satisfy the aforesaid conflicting demands of high oxidizing activity with respect to HC and CO, and suppression of sulfate, and in view of the difference of temperature and capacity between upstream and downstream, conventional oxidation catalysts for diesel engines therefore comprised for example a Pd type catalyst having a low oxidation activity and a separate Pt type catalyst of low capacity, i.e. having a high space velocity, installed in the upstream part of the exhaust.
This construction however fails to adequately resolve the following issues:
1. Even when the Pt catalyst is of low capacity, it produces a large amount of sulfate. PA0 2. As the Pd catalyst has a low oxidizing activity, it has only a low effect in reducing HC and CO. PA0 3. Catalyst performance cannot be maintained due to PM build up.